1. Field of the Invention
The present invention relates to a degaussing circuit for a CRT monitor, and more particularly, to a degaussing circuit for a CRT monitor stably performing a degaussing operation.
2. Description of the Related Art
A degaussing circuit restores magnetic properties of a cathode ray tube (CRT) so that images appear as clear images having original colors. Images may become blurred or changed to other colors by exposure to an earth magnetic field or other exterior circumstances.
FIG. 2 is a view illustrating a degaussing circuit of a conventional CRT monitor. As shown in FIG. 2, one end of a degaussing coil 111 is connected to a terminal of a power supply circuit part 20 through a Positive Temperature Coefficient (PTC) thermistor 115 and the other end of the degaussing coil 111 is connected to another terminal of the power supply circuit part 120 through contacts (1, 2) of a relay 118. One end of an actuator coil of the relay 118 is connected to a voltage source V10 through a resistor R1 and another end of the actuator coil is connected to a collector terminal of a pnp transistor 117. The actuator coil of the relay 118 is turned on when the pnp transistor 117 is turned on, thus closing the contacts (1, 2) of the relay 118. A diode D1 is connected in parallel with the actuator coil to dissipate energy stored in the actuator coil when the relay is turned off.
The pnp transistor 117 is turned on/off by a low/high degaussing control signal generated from a micom (not shown) when a power supply switch (not shown) of the CRT monitor is turned on or when a user wants to perform a degaussing operation. The low/high degaussing control signal is supplied to a base terminal of the transistor 117 through an input terminal 116 and a resistor R2. An emitter terminal of the transistor 117 is connected to a ground potential.
On degaussing, if a low signal is output from the micom to the pnp transistor 117, the pnp transistor 117 is turned on, to thereby operate the relay 118. When the relay 118 is operated, power is supplied to the degaussing coil 111 and current flows through the degaussing coil 111. When current flows in the PTC thermistor 115, an internal resistance of the PTC thermistor 115 is increased by a self heating and thus the current flowing through the degaussing coil 111 is cut off, to stop the degaussing operation.
However, according to the conventional degaussing circuit, even after the degaussing operation is stopped, a potential difference occurs by a residual current in the degaussing coil 111 and a magnetic field which exceeds safety standards for electric and magnetic field emissions is thereby created.